1. Field of the Invention
The present invention relates to an optical system which effectively performs anti-reflection on a lens surface while keeping a high optical performance, and is suitable for an optical equipment such as a video camera or a projector.
2. Description of the Related Art
There is a demand that an optical system used for an optical equipment such as a camera or a projector provide a high optical performance so as to meet the high performance of the optical equipment. In order to obtain the optical system with the higher optical performance, it is effective to use an aspherical lens in addition to a spherical lens. When reflected light from the lens surface enters an imaging plane to serve as ghost light, the image quality is largely deteriorated. Therefore, in order to obtain the optical system with the high optical performance, it is effective to provide an anti-reflection structure for preventing reflection on the lens surface.
An anti-reflection structure in which multiple structural parts are arranged at pitches shorter than the wavelength of visual light has been known as the anti-reflection structure low in reflectance in a wavelength region having a wide visible range and excellent in incident angle characteristic. There has been known an optical system in which such an anti-reflection structure is disposed on the lens surface to effectively reduce the reflected light (Japanese Patent Application Laid-Open No. 2005-62525 and Japanese Patent Application Laid-Open No. 2005-62526). Japanese Patent Application Laid-Open No. 2005-62525 discloses an optical system using an optical element in which a fine periodical structure having a period smaller than the wavelength of the incident light is formed on the aspherical surface. Further, Japanese Patent Application Laid-Open No. 2005-62526 discloses an optical system using an optical element with a fine periodical structure including a resin layer different in linear expansion coefficient from that of a base member in which the surface of the resin layer has a period smaller than the wavelength of the incident light.
When the aspherical lens is used as a part of the optical system, the correction of aberration is excellently performed, and the high optical performance is easily obtained over the entire screen. However, when the radius of curvature of the aspherical surface becomes smaller, an incident angle range of a light beam to the aspherical surface is enlarged, a large amount of unnecessary reflected light is generated from the aspherical surface, and the unnecessary reflected light enters the imaging plane, which causes flare and ghost to frequently occur. In particular, when the angle of field of the optical system increases, this tendency increases. The flare and ghost that occur on the aspherical surface are liable to be heteromorphous as compared with those occurring on the spherical surface. Because the occurrence of the heteromorphous ghost image is very conspicuous, such a ghost image is not preferable.
On the contrary, as compared with the anti-reflection structure formed of a thin film, the anti-reflection structure in which multiple structural parts each having a concavo-convex shape finer than the used wavelength are arranged on the lens surface provides the excellent anti-reflection effect in a wide incident angle range. However, in order to obtain the anti-reflection function (wavelength band characteristic) capable of reducing the reflected light from the aspherical surface in a wide wavelength range while enhancing the optical performance of the optical system by using the aspherical lens, it is important to appropriately set the shape of the aspherical surface that provides the anti-reflection structure.
In particular, in order to form the anti-reflection structure formed of multiple fine structural parts in the aspherical surface so as to effectively perform anti-reflection while ensuring the high optical performance, it is important to appropriately set the shape of the aspherical surface that provides the anti-reflection structure, and the applied portion thereof in the optical system. When those requirements are inappropriate, the excellent anti-reflection effect is not obtained, the heteromorphous ghost frequently occurs, and the optical system with a high optical performance is difficult to obtain.